1. Field of the Invention
The present invention relates to a time division duplex transceiver for alternately transmitting and receiving a signal having the same frequency.
2. Description of the Related Art
In conventional two-way radio communication systems, such as portable telephones, frequency division duplex (FDD) is often used. According to FDD, the radio communication system transmits a signal having a first frequency and receives a signal having a second frequency which is different from the first frequency. However, in recent years, since time-division multiplexing can be easily achieved with respect to a digital signal, time division duplex (TDD) has attracted considerable attention. According to TDD, the radio communication system alternately transmits and receives a signal having the same frequency. Compared with FDD, TDD is advantageous, for example, in that it is easy to allocate a frequency to each of a transmitting signal and receiving signal, and that transmitting and receiving efficiencies can be made substantially equal.
In a time division duplex transceiver, a transmitting frequency is identical with a receiving frequency. When a signal having the transmitting frequency is generated within the transceiver during the receiving operation of the transceiver, the signal is likely to enter the receiving section of the transceiver. This interferes with the receiving operation of the transceiver and deteriorates the receiving performance. Thus, during the receiving operation of the transceiver, it is necessary to completely terminate the generation of the signal having the transmitting frequency.
Furthermore, in cases where the transceiver is of a superheterodyne configuration and uses a local signal for frequency conversion in both of the transmitting and receiving sections of the transceiver, a transmitting intermediate frequency is identical with a receiving intermediate frequency. Thus, the above-described problem of deterioration in the receiving performance also arises when a signal having the transmitting intermediate frequency is generated during the receiving operation of the transceiver. In such a transceiver, therefore, it is necessary to completely terminate the generation of the signal having the transmitting intermediate frequency.
The generation of the transmitting signal can be prevented by terminating the operation of a transmitting local oscillator during the receiving operation of the transceiver. However, it is practically impossible to terminate the operation of the transmitting local oscillator during the receiving operation of the transceiver. This is because the local oscillator cannot be turned on and off rapidly enough to coincide with the high-speed alternation between transmission and reception in the transceiver. Thus, a common way of terminating the generation of the transmitting signal is as follows: During the receiving operation of the transceiver, with the transmitting local oscillator kept in operation, the signal line for the local signal or transmitting signal is electrically cut off, or a circuit which allows these signals to pass therethrough is disconnected from its power source, thereby terminating the generation of the transmitting signal. In this case, however, when the isolation of the electrically disconnected circuit is not sufficient, the generation of the transmitting signal cannot be completely terminated during the receiving operation of the transceiver. When the transmitting signal is generated during the receiving operation of the transceiver, the transmitting signal is likely to enter the receiving section of the transceiver. This interferes with the receiving operation of the transceiver, thereby deteriorating the receiving performance.
To prevent such deterioration in the receiving performance, there has been developed a time division transceiver including a local oscillator for producing a frequency of an integral multiple of a transmitting frequency which also includes a divider. For example, this type of transceiver is disclosed in Japanese Laid-Open Patent Publication No. 4-240924. In this transceiver, the output of the local oscillator is divided so as to obtain a signal having a transmitting frequency. During the receiving operation of the transceiver, with the local oscillator kept in operation, the divider is electrically disconnected from the local oscillator, thereby preventing the generation of the signal having the transmitting frequency.
In such a time division transceiver, however, the local signal from the local oscillator is required to have a frequency equal to or higher than twice that of the transmitting signal. When the transmitting signal has a higher frequency, it becomes more difficult to produce a stable local oscillator. This results in the cost for producing the transceiver to be high.